Electrical insulating oils are formulated so that they may meet or exceed certain specific, performance conditions. These conditions include a minimum pour point, a maximum kinematic viscosity and enumerated limits on interfacial tension, impulse breakdown strength, gassing tendency and levels of acid number and sludge produced in oxidation tests.
Current commercial practice is to use naphthenic distillates as the basestock for electrical insulating oils. Typically the basestock is combined with an effective amount of an antioxidant additive, commonly a hindered phenol. Examples of such antioxidants include 2,6di-t-butyl phenol and 2,6-di-t-butyl cresol. Use of oxidation inhibitors, however, is limited. ASTM D3487 describes Type I oils as being restricted to a maximum of 0.08 wt % oxidation inhibitor and Type II oils, 0.3 wt % inhibitor.
As electrical equipment manufacturers develop more efficient electrical devices there will be a need for electrical oils that have better oxidation resistance than the current oils based on naphthenic basestocks. Higher oxidation resistance can be achieved by use of paraffinic basestocks; however, paraffinic basestock exhibit what is referred to as a positive gassing. The gassing tendency of an oil is a measure of the rate at which hydrogen gas is either evolved or absorbed in an insulating medium when that medium is subjected to electrical stress sufficient to cause ionization. A positive gassing tendency indicates that hydrogen gas is given off, while a negative gassing tendency indicates that hydrogen gas is absorbed. A negative gassing tendency, or very low positive tendency, is desirable since it will minimize the build-up of hydrogen gas which could react with oxygen in the presence of a discharge spark to cause an explosion in the electrical device. Insulating oils shown to have gas absorbing characteristics have been used to advantage in reducing equipment failure, particularly in cables and capacitors.
The gassing tendency of electrical oils is measured by test method ASTM D 2300. Oils that evolve hydrogen gas have a positive test value and those that absorb hydrogen gas have a negative test value.